In general, the task of computing an amplitude is to reduce it to a
linear combination of known scalar integrals. Using standard
integral-reduction techniques (see e.g. Refs. [1]}, [2]}) any four-point one-loop amplitude can be written as
a linear combination of box, triangle, bubble and tadpole integrals,
\(\mathcal {M}_4^\text{1-loop} = \Bigl (d_{s,t} I_4(s,t) + c_s I_3(s) + b_s I_2(s) + \hbox{perms.} \Bigr )+ b_0 I_2(0) + a_0 I_1\,,\) 
